vSphere products
distinguish between upgrades, which make major changes to the software, and
updates, which make smaller changes to the software.

VMware product versions are
numbered with two digits, for example, vSphere 5.1. A release that changes
either digit, for example, from 4.1 to 5.0, or from 5.0 to 5.1, involves major
changes in the software, and requires an upgrade from the previous version. A
release that makes a smaller change, requiring only an update, is indicated by
an update number, for example, vSphere 5.1 Update 1.

When you upgrade an ESXi host,
some host configuration information is preserved in the upgraded version, and
the upgraded host, after rebooting, can join a vCenter Server instance that has
been upgraded to the same level. Because updates and patches do not involve
major changes to the software, host configuration is not affected.